The invention relates to a method for operating an internal combustion engine, in particular a gas engine, having at least two cylinders. A cylinder-specific first cylinder signal is acquired from each cylinder, and at least one combustion parameter of the corresponding cylinder is controlled as a function of the first cylinder signal.
The cylinders of an internal combustion engine normally exhibit technical differences in combustion, i.e. when combustion parameters such as the quantity of fuel or the ignition point are controlled in an overall manner, the individual contributions by the cylinders to the total work carried out by the internal combustion engine are different. The term “overall control” or “overall engine control” of combustion parameters as used in the context of the invention means that all of the cylinders of an internal combustion engine are operated with the same values for the corresponding variables, i.e., for example, for overall control as regards fuel quantity, the same open period is applied to the gas injection valves for each cylinder, or for overall control as regards the ignition point, the ignition devices of the cylinders are each activated at the same piston position of the respective piston in the cylinder—normally expressed as the crank angle before TDC (top dead center of the piston in the cylinder).
The work of a cylinder in a reciprocating engine is transmitted via a crankshaft connected to a connecting rod of the cylinder to an output shaft of the internal combustion engine. Frequently, an electrical generator is connected to the output shaft in order to convert the mechanical energy of the output shaft into electrical energy. Of the various possibilities for cylinder balancing, focus is on balancing the peak pressures in the individual cylinders in order to obtain as even as possible a mechanical peak load on the components. Examples of major alternative balancing variations are optimizing the engine efficiency or minimizing pollutant emissions.
Having regard to cylinder balancing control, U.S. Pat. No. 7,957,889 B2 describes tailoring the introduction of fuel for each cylinder of an internal combustion engine such that the maximum internal cylinder pressure or peak cylinder pressure of each cylinder is set to a common target value with a tolerance band. The target value in that case is obtained from the arithmetic mean of all of the peak cylinder pressures.
The systems described until now use the arithmetic mean of cylinder-specific signals such as the peak cylinder pressure as the target variable for cylinder balance control. However, those systems do not take into account the cylinder-specific differences which arise, for example, from cylinder parameters such as air charge, deposits and wear, center of combustion, or mechanical tolerances. These, in particular, give rise to scatter in the emissions and combustion properties, which can lead to loss of efficiency. In this regard, cylinders with lower nitrogen oxide or NOx emissions can lose more efficiency than cylinders with higher NOx emissions can gain. Since overall—i.e. taken over the whole internal combustion engine—certain NOx limits often have to be complied with, such a spread in the NOx emissions from the individual cylinders often results in an overall loss of efficiency of the internal combustion engine due to the cylinder-specific differences in the cylinder parameters.